Displays of the type to which this disclosure finds application include multiple color image projection systems that employ two or more spatial light modulators (SLMs), such as Texas Instruments DLP® digital micromirror device (DMD) light modulators. The architecture and operation of such micromirror devices is described in more detail in Texas Instruments Application No. 61/823,304 filed May 14, 2013, entitled “Micromirror Apparatus and Methods,” the entirety of which is incorporated herein by reference.
Multiple DMD (3-chip typical) systems using conventional +/−12-degree (“ON”-state/“OFF”-state) tilting mirror pixels arranged in a Manhattan-type array use a Philips-type color separation prism (“Philips prism”) to split white light from an illumination source into constituent RGB color components to respective DMDs, and to recombine the separately modulated colors into a composite complete RGB image for projection onto a display surface. Changes in the illumination angle vs. projection angle on the prism dichroic color filters results in spectral losses in the output, and separation of s- and p-polarizations on the dichroic color filters. A single total internal reflection (TIR) prism is used between the projection lens and the Phillips prism to inject the light at a 45-degree orientation to the DMD's, therefore each DMD has to be oriented to the light path in the same direction. Since tow of the three colors will split and/or recombine by reflection from the (normally) green straight-thru path, those reflected colors (red and blue) have to undergo an even number of reflections (typically two) to maintain correct orientation of the incoming light to the DMDs. Thus the red and blue paths are significantly longer in a DMD projection system Philips prism compared to, for example, an x-cube type prism used in other technologies. Also, in order to provide a same path length in the glass for all colors, the green path may become relatively much longer in order to match the red and blue paths. Since the outgoing projected light must pass again through the TIR prism at the front and then into the projection lens, the cones of light coming into the TIR prism from the long path of the Phillips prism have now expanded significantly since leaving the respective DMDs. This makes the TIR prism significantly larger than desirable for the illumination path, to prevent vignetting of the projected image bundles. Also, the illumination bundles in the prism are folded about the diagonal dimension of the DMD due to the corner-Manhattan pixel architecture having a diagonal hinge. This increases the size of the prisms. And since color-splitting (illumination path) and color-combining (projection path) occurs in the same space in the same prisms, there are significantly more stray light paths to manage, which can increase the path lengths required. The net result is that the projection path length in glass for the prism sets for multi-chip DMD projectors using corner-Manhattan array layout devices may be significantly longer than required for some other technologies, resulting in significantly longer back-working-distance requirements for the projection lenses. This has an impact on the resulting size, performance, and cost of the projection lenses. It unnecessarily complicates the design by requiring retro-focus lens designs in which the focal length of the lens is significantly shorter than the back working distance of the lens, requiring more complicated designs with more lens elements which may degrade performance and/or increase cost.
A conventional x-cube type prism is not practical for current multi-chip DMDs for two reasons. First, the number of reflections for blue and red channels is odd (one instead of two), requiring a different orientation of the DMD relative to the incoming light, which is at an angle to the optical axis. This is difficult to implement with a single TIR prism feeding the input light to the color splitting prism. And, second, typical x-cube dichroics only have to deal with one polarization of light for other technologies, so the coatings do not have to minimize separation of the s- and p-polarizations for color fidelity since there is only one polarization to begin with.
In DMD projection systems, the light is randomly polarized, so the s- and p-polarization split may cause light from one or another of the polarized components to leak across the filter, thereby contaminating the color purity of the other channels. For instance, some blue light may leak into the green channel, some red light may leak into the green channel, and some green light may leak into the red and blue channels, which reduces the color fidelity and gamut. This separation effect may be minimized with higher cost dichroic coatings, but not totally eliminated.
A smaller, lower cost solution would enable lower-cost, smaller-screen cinema markets as well as high performance, economical multi-chip home theatre and professional projection applications.